1. The Field of the Invention
The present invention relates to the field of fiber optic couplers, and more specifically, to polarization maintaining fiber optic couplers that provide for an adjustable coupling ratio.
2. The Relevant Technology
The field of fiber optic communications has enjoyed rapid growth over the last decade. As data transfer rates increase, more and more information is being compressed into currently available fiber optic lines. This need for speed has resulted in a corresponding need for equipment to precisely and accurately transmit large amounts of data over great distances with little or no data loss. One method of maximizing the efficiency of transmission is to transmit signals with a well maintained state of polarization.
As light signals propagate over long distances, the signals will be attenuated due to losses in the optical fiber such as scattering losses and absorption losses. Reduced signal strength often results in high bit error rate, a significant system impairment. To counter signal propagation losses in the optical fiber, optical amplifiers are often used to boost the signal strength. One popular optical amplifier is a Raman amplifier, which has a very low signal to noise ratio and reasonable amplification (20 dB). Since Raman amplification has a polarization dependent gain, a polarization controller is often used to combine two orthogonally polarized pumps for the laser light. Optical couplers, polarization beam splitters and combiners, and polarization maintaining (PM) fibers are all useful for this purpose.
A cross-section of an exemplary prior art PM fiber is illustrated in FIG. 1, and designated generally as 100. PM fiber 100 has a core 102 surrounded by a cladding 104, which is further covered in a protective sheath 106. Stress rods 108 are located on either side of core 102 within cladding 104. An optical axis 110, passing through stress rods 108, cladding 104 and core 102, indicates one direction of the polarization maintaining axis for PM fiber 100. The other polarization maintaining axis is perpendicular to axis 110. PM fiber 100 works by placing stresses on fiber core 102, creating two perpendicular transmission axes. If linearly polarized light is input into fiber 100 along one of these axes, the polarization state is maintained for the length of fiber 100. There are various types of PM fibers widely available in the market today, including panda, bow-tie, as shown in FIG. 1, and tiger.
PM fibers are often used in an angled physical contact (APC) connector. APC connectors have fiber ends that are terminated and polished at an angle. This angle can be anywhere from a few degrees to about 15 degrees. When compared with a normal physical contact (PC) connector, which has a dome shaped endface, an APC connector exhibits better reflectance properties, because the angled and polished end surface reduces the amount of light reflected back into the fiber at the connector interface. There are various types of connectors available with an angled and polished endface including, for example, SC, ST, FC, LC, MU, MT, and MTP™.
A fiber optic coupler is an optical component that either splits light coming from one input optical fiber into two or more output optical fibers or combines two or more input beams into a single output beam. Polarization maintaining couplers use PM optical fibers. These couplers and fibers split or combine light signals according to the state of polarization of the light passing through them. PM couplers can be used in an optical communications system, such as a Raman amplifier, to provide amplification to the optical signal. PM couplers can also be used as multiplexers and de-multiplexers to combine signals from different fibers into one fiber or split signals from one fiber to other fibers. In one application, a PM coupler is used to combine two light beams input from different optical fibers into a single optical fiber. This type of coupler can be known as a polarization beam combiner. Alternately, a polarizing beam splitter is a coupler that takes an input light beam, splits it into two orthogonally polarized beams, and outputs oz each of these beams into a separate optical fiber.
A basic design for a system employing a polarizing beam splitter is shown in FIG. 2 and designated generally as 200. Input fiber 202 transmits an input light beam into polarizing beam splitter (PBS) 204. The input beam can be a beam with any state of polarization, such as a beam that has traveled some distance along a single mode fiber. Alternately, the beam can be a polarized beam from a PM fiber. In either case, the input beam is split into two orthogonally polarized output beams in PBS 204.
The two output beams can be polarized, for example, in the horizontal and vertical directions, respectively. They can then be received by respective PM fibers 206 and 208. PBS 204 can send an input beam either to PM fiber 206 or PM fiber 208, or to both of them, depending on the polarization state of the input beam, i.e. whether a beam is oriented with horizontal or vertical polarization axes. For example a horizontally polarized input beam would be directed by PBS 204 into PM fiber 208, while a vertically polarized input beam would be directed by PBS 204 into PM fiber 206. For an input beam with a polarization direction in between, the input beam will be split into two beams having orthogonal polarization, which are directed into PM fibers 206 and 208. Each beam is a projective component of the input polarization light onto the vertical and horizontal axes.
Light input from a single mode fiber can have random polarization. The amount of light that is transmitted into each of the output fibers is determined by the state of polarization of the input light. The relationship between the input and output is known as the coupling ratio. Coupling ratio or splitting ratio can be defined as the ratio of the optical power from one output port of the coupler to the sum of the total power from all output ports. The coupling ratio of an output fiber can be measured at a center wavelength and is often expressed as a percentage as follows:
                    R        =                  100          ⁢                                          ⁢                                    P              x                                                      ∑                                  i                  =                  0                                n                            ⁢                              P                i                                                                        (        1        )            where R is the coupling ratio, Px is the power in any one output fiber and the denominator is the sum of the power in all of the output fibers.
One problem with the system described above is that changing the coupling ratio to generate a specific amount of power in a given output fiber can be very difficult. It requires that additional equipment or optical elements be included, thus greatly increasing the cost of the system.